As described above, it is often important that an optical system using an image pickup device be focused for optimum use of the image pickup device. When the image pickup device is used in performing remote control measurement or in performing pattern recognition, measured values or results of judgment are directly affected by whether the optical system is in its focused state or not, and therefore it is important to have the optical system in its focused state accurately prior to use. In the case of a camera, an object to be picked-up may be viewed through a finder and focused by the visual sense of a person. However, the visual sense is not desirable because there may be an individual difference in judgment as to whether the object to be picked-up is focused or not. There has been proposed a method of adjusting an optical system so as to agree with a numerical value of the distance measured by using a range finder. However, there is a possibility that the optical system gets out of order as the time elapses and therefore it is still necessary to ascertain directly whether the focused state has been obtained or not. In the case of measurement, or the like, a video signal obtained from an image pickup device is processed or utilized in any case and it is therefore most desirable in the measurement to judge whether the focused state has been obtained or not on the basis of the video signal.
In judging a focused state on the basis of a video signal, one can expect that the focused state has been obtained when the video signal shows the steepest leading or trailing edge in a portion where there is a difference in hue or in distribution between a bright and a dark portion in an object to be picked-up by utilizing the fact that the image becomes unclear when the focus is shifted. Similarly there may be utilized a difference in brightness and/or hue between the object to be picked-up and the background. It is possible to inspect waveforms showing the leading and trailing edges of a video signal by using a synchroscope or the like and it is also possible to perform focus adjustment while inspecting the waveforms obtained with such a measuring instrument. However, a relatively expensive measuring instrument and technical skill for using such an instrument are required and the inspection of the waveform itself depends on a visual sense of a person so that it is impossible to avoid an individual difference to a certain extent.
Accordingly, attempts have been made to do the focusing electrically, eliminating subjective judgments. However, it has been characteristic of prior art techniques of this kind that they have tended to be sensitive to the illumination level of the object and have tended to depend heavily on threshold settings determined by the expected levels of illumination of the background and object.
An object of the present invention is therefore to overcome such difficulties in the prior art and to obtain a novel method for detecting focusing which is little affected by variations in intensity of illumination received by an object to be picked-up and which employs equipment which is relatively simple.
A secondary object of the invention is to obtain a novel method for detecting focusing in which detection of focusing can be performed accurately even in the case where contrast in quantity between the light from an object and the light from a background and/or a difference in brightness and/or hue in the object are small.
A related object is apparatus for performing this novel method.